A 3-D nanoribbon-like Pt-free oxygen reduction reaction electrocatalyst derived from waste leather for anion exchange membrane fuel cells and zinc-air batteries

Hierarchical Porous Fe3C@Fe-N-C Catalysts from Tannin-Fe(III) Complexes for Efficient Oxygen Reduction

The rational design of metal-nitrogen-doped carbons (M-N-C) from available and cost-effective sources featuring high electrocatalytic performance and stability is attractive for the development of viable low-temperature fuel cells. Herein, mimosa tannin, an abundant polyphenol easily extracted from the Mimosa plant, is used as a natural carbon source to produce a tannin-Fe(III) coordination complex. This process is assisted by Pluronic F127, which acts as both a surfactant and a promoter of Fe-Nx active sites. After carbonization in the presence of urea as a nitrogen precursor, this organic tannin-Fe(III) framework produces Fe3C nanoparticles encapsulated on a Fe-N-C single-atom catalyst with hierarchical porosity. The optimal catalyst, with a Pluronic F127/mimosa tannin mass ratio of 0.5, exhibits high ORR performance in both alkaline and acidic electrolytes, with half-wave potentials of 0.87 and 0.74 V versus RHE, respectively. In addition, good performance is achieved in practical hydrogen polymer-electrolyte membrane fuel cells using OH-- or H+-conducting membranes with peak power densities of 242 and 200 mW cm-2 at cell voltages of 0.43 and 0.3 V, respectively. The synthetic approach can be explored to design new renewable M-N-C electrodes for electrochemical energy conversion or storage devices due to tannin's exceptional ability to coordinate metals.

Carbonized Leather Waste: A Review and Conductivity Outlook

The carbonization of collagen-based leather waste to nitrogen-containing carbon is reviewed with respect to the preparation, characterization of carbonized products, and applications proposed in the literature. The resulting nitrogen-containing carbons with fibrous morphology have been used as adsorbents in water pollution treatment, in electrocatalysis, and especially in electrodes of energy-storage devices, such as supercapacitors and batteries. Although electrical conductivity has been implicitly exploited in many cases, the quantitative determination of this parameter has been addressed in the literature only marginally. In this report, attention has been newly paid to the determination of conductivity and its dependence on carbonization temperature. The resulting powders cannot be compressed into pellets for routine conductivity determination. A new method has been used to follow the resistivity of powders as a function of pressure up to 10 MPa. The conductivity at this pressure increased from 9.4 × 10^-8 S cm^-1 for carbonization at 500 °C to 5.3 S cm^-1 at 1000 °C. The conductivity of the last sample was comparable with conducting polymers such as polypyrrole. The carbonized leather thus has the potential to be used in applications requiring electrical conduction.

Atomically Dispersed Iron with Densely Exposed Active Sites as Bifunctional Oxygen Catalysts for Zinc-Air Flow Batteries

Atomically dispersed iron embedded carbon is a promising bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), but its exposed iron sites must be increased. Herein, the authors propose a double steric hindrance strategy by using zeolitic imidazolate frameworks-8 as the first barrier skeleton and encapsulated phenylboronic acid as the second space obstruction to realize densely exposed atomic iron sites. Prepared PA@Z8-FeNC has the highest iron content (5.49 wt%) among reported transition-metal-based single-atom oxygen catalysts. Meanwhile, its concave surfaces, hollow structures, and hierarchical pores enable the high utilization rate of iron sites to 88.5 ± 4.5% and exposed active site density to 5.2 ± 0.3 × 10²⁰ sites g^-1 . Resultantly, PA@Z8-FeNC exhibits superior activity and stability to commercial Pt/C and IrO₂ for the ORR and OER in half-cells and zinc-air flow batteries. This provides insight for developing densely and accessibly active sites in single-atom catalysts.

The mechanism of Co oxyhydroxide nano-islands deposited on a Pt surface to promote the oxygen reduction reaction at the cathode of fuel cells

With the rapid development of fuel cell technology, the low reduction rate of oxygen on Pt-based cathodes is generally considered the main obstacle. Pt/transition metal alloys (Pt-Ms) or Pt/transition metal oxides (Pt-MO _x ) can be formed by doping transition metal atoms into the lattice of the Pt layer or depositing onto the surface of the Pt layer to intensify the catalytic activity of the electrodes. In this work, a stepwise solution chemical reduction method for high dispersion of cobalt oxyhydroxide (-OCoOH) deposited onto the facet of Pt as nano-islands and the mechanism of promoting the oxygen reduction reaction (ORR) at the cathode have been investigated by density functional theory (DFT) calculation. As a result, the electrocatalytic activity of Pt with nano-island -OCoOH structure was 3.6 times that of the Pt/C catalyst, which indicated that promoting the desorption of the first O atom and weakening the adsorption capacity of the interfacial junction Pt for the second O atom from adsorbed oxygen attributed to the migration of d-band center in Pt and the existence of the Co hydroxyl group.

Prospects for Anion-Exchange Membranes in Alkali Metal–Air Batteries

Rechargeable alkali metal–air batteries have enormous potential in energy storage applications due to their high energy densities, low cost, and environmental friendliness. Membrane separators determine the performance and economic viability of these batteries. Usually, porous membrane separators taken from lithium-based batteries are used. Moreover, composite and cation-exchange membranes have been tested. However, crossover of unwanted species (such as zincate ions in zinc–air flow batteries) and/or low hydroxide ions conductivity are major issues to be overcome. On the other hand, state-of-art anion-exchange membranes (AEMs) have been applied to meet the current challenges with regard to rechargeable zinc–air batteries, which have received the most attention among alkali metal–air batteries. The recent advances and remaining challenges of AEMs for these batteries are critically discussed in this review. Correlation between the properties of the AEMs and performance and cyclability of the batteries is discussed. Finally, strategies for overcoming the remaining challenges and future outlooks on the topic are briefly provided. We believe this paper will play a significant role in promoting R&D on developing suitable AEMs with potential applications in alkali metal–air flow batteries.

Graphite N-C-P dominated three-dimensional nitrogen and phosphorus co-doped holey graphene foams as high-efficiency electrocatalysts for Zn-air batteries

The search for metal-free catalysts for oxygen reduction reactions (ORRs) in energy storage and conversion devices, such as fuel cells and metal-air batteries, is highly desirable but challenging. Here, we have designed and synthesized controllable 3D nitrogen and phosphorous co-doped holey graphene foams (N,P-HGFs) as a high-efficiency ORR catalyst through structural regulation and electronic engineering. The obtained catalyst shows a half-wave potential of 0.865 V in alkaline electrolytes. It is found that Zn-air batteries with the N,P-HGFs-1000 air electrode exhibit excellent discharge performance and durability. Our study suggests that the remarkable ORR performance of N,P co-doped graphene is mainly due to the graphite N-C-P structure, where an enhanced charge density and increased HOMO energy level are confirmed by both experimental results and theoretical density-functional theory calculations.